medium · SAT Reading & Writing

Astronomers have long puzzled over the 'Oort Cloud,' a theoretical shell of icy objects at the outermost edge of the solar system. While no direct observations of the cloud exist, researchers infer its presence from the trajectories of long-period comets, which enter the inner solar system from all directions. This isotropic distribution suggests that the source must be a spherical shell rather than a flat disk, like the Kuiper Belt closer to the Sun. Which choice best states the main idea of the text?

1. Long-period comets are the only objects in the solar system that follow isotropic trajectories.
2. Scientists use the movement of certain comets to deduce the shape and existence of a distant, unobserved region of space.
3. The Oort Cloud and the Kuiper Belt are identical in their physical composition and geometric shape.
4. Direct observations of the Oort Cloud have confirmed that it consists entirely of icy objects.

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